How would you possibly manipulate sf::Clock with this?
Your question makes no sense. You don't manipulate sf::Clock. You manipulate the object of type sf::Clock named mClock1.
It's a class member, so you first have to make an instacne of the class MyClass someInstance;
and then you can do things with its members doSomething(someInstance.mClock1);
What would be the point of this?
What's the point of classes? Well, sometimes in programming you find that the built in data typers, such as int and double, don't do everything you need. So you define your own new kinds of objects. In C++, we call these classes (or structs).